Although, recycling of paper saves energy, reduces pollution, preserves trees and conserves landfill space, yet the same is a complicated and disordered process as it involves the use of caustic chemicals and produces harmful by-products and emissions. The paper recycling process requires the removal of printing inks from the used paper, which is really problematic because it is not really ink but rather a plastic polymer which the printer or copier burns onto the paper. The removal of these inks requires chemicals that are much more corrosive than standard de-inking chemicals. Moreover, printing inks contain heavy metals and other compounds that require strong solvents to remove these compounds. Furthermore, when recycling facilities remove inks from paper, the waste, which includes the metals used in printing inks, such as copper, lead, zinc, chromium and cadmium makes its way into the water stream. Therefore, the paper recycling industries are struggling to devise more eco-friendly and greener techniques so as to eliminate the sludge formed during the de-inking process.
In light of the above, the best way to reduce paper-related pollution and consumption of energy is to cut back on paper consumption, which will decrease the demand for new or recycled paper. However, this is not possible due to the increase in printing on paper owing to the global civilization. In most of the cases, paper is printed to serve temporary jobs, for example, daily newspapers and after the consumption, it becomes useless and is thrown away as waste paper. Thus, tons of papers are wasted each day at the cost of thousands of trees, causing global deforestation and lots of associated environmental problems. As mentioned above, the conventional way to reduce the paper waste is to recycle the used paper and produce fresh papers; however, the chemicals used for this recycling process should be environmentally and economically acceptable.
Therefore, inkless and erasable printing appears to be an important and key solution for reducing the deforestation and related problems arising from the global paper manufacture and recycling. Photochromic materials are capable of changing colour when exposed to light and such photochromic materials contain spiropyran, diarylethene and azobenzene or redox active cores in their structures which are responsible for the colour change when exposed to light. These photochromic materials have been extensively used for making photochromic glasses, lenses, and filters because of their interesting reversible colour change property. Moreover, these materials have also been proposed for applications like erasable and inkless printing, 3D data storage, etc. However, conventional photochromic materials, as mentioned above, have short lifetime and return to the initial colour within a few minutes of excitation, making them inappropriate for the inkless erasable printing applications.
As is evident from the foregoing, the prior art fails to address the following necessary conditions which are essential to design a practical erasable printing media which include: (i) their ability to retain the photogenerated colour for a prolonged period of time so that the content remains legible/readable; (ii) the reversibility of this colour change so that the same paper can be used for multiple cycles, and (iii) the intactness of the colour in presence of paper contents.
Article titled “A novel photochromic calcium-based metal-organic framework derived from a naphthalene diimide chromophore” by L Han et al. published in Chem. Commun., 2013,49, pp 406-408 reports a novel 3D calcium-based metal-organic framework based on a naphthalenediimide chromophore which displays a unique doubly interpenetrated 7-connected net with total point symbol of {36·49·56}. The MOF has excellent thermal stability and reversible photochromic properties varying from yellowish to dark green. The Compound [Ca2(BIPA-TC)(DMF)4].2DMF was obtained as yellowish block crystals via a solvothermal reaction of H4BIPA-TC and Ca(NO3)24H2O in DMF at 100° C. for 72 h. However, the article does not describe any attempt to use the photochromic property of the materials as a medium for inkless and erasable printing.
Article titled “Alkali earth metal (Ca, Sr, Ba) based thermostable metal-organic frameworks (MOFs) for proton conduction” by T Kundu et al. published in Chem. Commun., 2012, 48, pp 4998-5000 reports three new alkaline earth metal based MOFs that have been synthesized by using 4,4′-sulfobisbenzoic acid (SBBA) and alkaline earth metal salts M(NO3)2, M=Ca, Sr, Ba. These MOFs exhibit interesting structural diversity, variable chemical stability as well as proton conductivity. However, the article doesn't deal with any photochromic property of the materials (MOFs), nor with the application as inkless and erasable printing.
Article titled “Structure Design of Naphthalimide Derivatives: Toward Versatile Photoinitiators for Near-UV/Visible LEDs, 3D Printing, and Water-Soluble Photoinitiating Systems” by J Zhang et al. published in Macromolecules, Apr. 3, 2015, 48 (7), pp 2054-2063 reports seven naphthalimide derivatives (NDP1-NDP7) with different substituents as versatile photoinitiators (PIs), and some of them when combined with an iodonium salt (and optionally N-vinylcarbazole) or an amine (and optionally chlorotriazine) are expected to exhibit an enhanced efficiency to initiate the cationic polymerization of epoxides.
Article titled “Patterned Deposition of Metal-Organic Frameworks onto Plastic, Paper, and Textile Substrates by Inkjet Printing of a Precursor Solution” by J L Zhuang et al. published in Adv Mater., 2013; 25(33), pp 4631-4635 reports that inkjet printing of metal-organic frameworks permits their larger area, high-resolution deposition in any desired pattern, even in the form of gradients or shades.
U.S. Patent application no. 20080311495 discloses a photochromic material, a substrate, methods, and apparatus for inkless printing on reimageable paper. The document discloses an inkless reimagable paper or image forming medium formed using a composition that is imagable and erasable by heat and light, such as comprising as a photochromic material a spiropyran compound having a conjugated pathway, dispersed in a polymeric binder, wherein the photochromic material exhibits a reversible transition between a colorless state and a colored state in response to heat and light.
U.S. Patent application no. 2006251988 discloses an image forming medium including at least a polymer and a photochromic compound such as spiropyran embedded in the polymer, wherein spiropyran molecules of the spiropyran compound are chelated by a cation. The document also discloses a method of manufacturing an image forming medium comprising, dissolving an amount of a photochromic compound, which contains chelating groups, in a solvent; adding a salt, which chelates the photochromic compound, to the solvent; adding a polymer to the solvent; and mixing the solvent until the photochromic compound and the polymer are dissolved in the solvent, then providing a substrate; providing a layer of the mixed solvent on a first surface of the substrate; drying the substrate; and providing a background on a second surface of the substrate opposite the first surface, wherein the solvent comprises approximately 10% polyvinyl alcohol and approximately 2-5% ethanol in distilled water.
European patent no. 2832767 discloses a method for preparing covalent organic frameworks (COFs) and uses of the obtained COFs in the coating surfaces, ink-jet printing, spray deposition, material encapsulation and coordination chemistry.
Consequently, there is an urgent need in the art for a suitable photochromic material which can fulfill all the above mentioned requirements and can be used for practical applications in inkless and erasable printing media thereby making the recycling paper industry greener and environmentally cleaner.